This invention relates to an image forming apparatus for developing an electrostatic image or a magnetic latent image in an electrophotographic method, an electrostatic printing method, or a magnetic recording method. In particular, this invention relates to a tandem type color image forming apparatus which is provided with a plurality of image-forming units.
Due to increasing trends to enhance the quality of image, to micronize the particle size of toner to employ, and to enhance the sphericity of toner in the tandem type color image forming apparatus, it is now becoming increasingly difficult to perform the cleaning of photoreceptor by means of blade. In order to miniaturize the tandem type color image forming apparatus where image-forming stations are juxtaposed horizontally with each other in particular, the distance between these stations is required to be shortened as much as possible since the number of image forming apparatus would be four for instance.
Under the circumstances, a cleaner-less process where a cleaner such as the aforementioned blade can be dispensed with is attracting many attentions in recent years.
The tandem type color image forming apparatus is accompanied with a problem of “reverse transcription”, i.e. a phenomenon wherein an image that has been transferred to paper or an intermediate transfer body is caused to partially adhere onto a photoreceptor of later stage when the image happens to correspond with a non-image portion in the transfer region of a station of later stage. Since no cleaning means is provided before the transferred image is moved to the developing device in the cleaner-less process in particular, this reversely transferred toner is recovered in a developing device of later stage and mixed with a developing agent to be employed in a process of later stage, thereby generating a phenomenon of so-called color mixing wherein the color tone of image is caused to gradually change. Although the degree of color mixing is caused to fluctuate depending on the kind of image to be printed as well as on the kind of pattern, this color mixing is a fundamental problem inherent to the cleaner-less process of the tandem type color image forming apparatus.
In order to cope with this reverse transcription, there has been proposed a method, as shown in JP Laid-open Patent Publication (Kokai) No. 2000-242152 for example, wherein a roller member for removing the developing agent which has been reversely electrified, e.g. a brush roller, is contacted with a photoreceptor after finishing the transfer of image and applied with a bias voltage of the same polarity as the electrification polarity of the normal toner, thereby selectively recovering only the toner that has been electrified with reversed polarity. Although it is possible, according to this method, to prevent the color mixing resulting from the reverse transcription in the cleaner-less process, since it is required to mount a brush roller at the same position where the conventional blade cleaner is to be mounted, the resultant structure would become complicated, thus raising the problems that it is impossible to miniaturize the image forming apparatus and to reduce the manufacturing cost of the image forming apparatus.
Further, there is also proposed a method as disclosed in JP Laid-open Patent Publication (Kokai) No. 2004-93849 wherein a temporary retaining member such as a brush roller is employed and the switching of bias voltage to this temporary retaining member is performed in such a manner that during the printing operation of image, only the reversely electrified developing agent is recovered and retained in the temporary retaining member and during the non-printing operation, the developing agent recovered in the temporary retaining member is released to the photoreceptor.
It may be possible, through the application of this method to the tandem cleaner-less system, to recover the reversely transferred toner. However, since the reversely transferred toner is put back to the photoreceptor and ultimately recovered in the developing device, there is still a problem that it is impossible to avoid the color mixing.
Further, there is also proposed a method as disclosed in JP Laid-open Patent Publication (Kokai) No. 4-20986 (1992) wherein an electrification/disturbance member is employed to disturb the memory retained in the residual toner which is left untransferred concurrent with the electrification of toner for the next step in the cleaner-less process. However, since the reversely transferred toner is also recovered in the developing device together with the residual toner, it is impossible to avoid the color mixing.